The invention relates to an apparatus and process for cryogenic separation of air which uses at least one rectifying column. The rectifying column comprises at least one section equipped with packing materials, and a liquid distributor positioned above this section. In addition, the invention relates to a liquid distributor suitable for use in a mass transfer column in which material is exchanged between a liquid phase and a vapor phase.
Air separation units are used to recover oxygen, nitrogen and/or noble gases from air. Their basic design and their mode of operation are known, for example, from the monograph "Tieftemperaturtechnik Cryogenics!" by Hausen/Linde (2nd edition, 1985) or from an article by Latimer in Chemical Engineering Progress (Vol. 63, No. 2, 1967, page 35). For some years, packing materials, both nonstructured and structured, especially structured packings, have typically been used in rectifying columns of air separators. Nonstructured packings are dumped packings such as, for example, Rashig rings, Pall rings, Berl saddles, Intallox saddles and the like. Structured packings are arranged packings such as, for example, KochSulzer packings, Flexipac and the like. See also Perry's Chemical Engineer's Handbook, 6th edition, pp. 18-19 to 18-41 (1984). The effectiveness of the packing materials greatly depends on uniform wetting of their surface with reflux liquid. Therefore, there is placed, above a structured packing section, a liquid distributor to distribute the liquid flowing downward as homogeneously as possible over the column cross-section.
In many cases, air separation units are not run with constant throughput even in normal operation (in this regard, throughput refers to the volumetric flow rate of feed rate of air introduced into the air separator). This is especially the case when the resultant product is fed directly to another process that is not operated continuously or at a steady state, but, for example, is operated periodically. In other situations, the need exists to occasionally change the throughput of the air separator unit. The same problem of changes in the throughput occur especially when starting up each air separation unit.
If the air volume introduced into an air separator unit is reduced, for example, some time will pass until the steady equilibrium states corresponding to the new load are achieved at all points within the air separation column(s). As a result, in previously known air separation processes and units, brief load changes by changing column conversion, i.e., production amounts, are practically not possible.
Rectifying columns of air separators react extraordinarily sluggishly to such load variation, so that narrow limits are placed on the flexibility of oxygen and nitrogen production. Further, losses in yield and purity will result in cases of load variations.
With the recently introduced use of structured packings, instead of conventional exchange plates, certain improvements in start-up behavior and flexibility relative to load variations have been achieved because structured packings have a significantly smaller liquid holdup than plates. However, packed columns still have an unsatisfactorily high sluggishness in responding to load changes.
In the past, processes were developed for special applications that made it possible to vary production volumes with a constant loading of the rectifying columns. W. Rohde describes a special air separation process with variable storage adapted to handle very great fluctuations in "LINDE-Berichte aus Technik und Wissenschaft" (LINDE Reports on Technology and Science), Vol. 54 (1984), pp. 18-20. In this case, product oxygen is fed directly to a periodically operating oxygen-blowing process in the steel industry. The very high fluctuations in product volumes of the air separation system which occur in this example, over 40% of the maximum capacity of the air separator, are compensated for by buffer tanks, and the throughput through the rectifying columns can be kept essentially constant. However, such units with buffer tanks are very expensive in equipment and control engineering, especially with smaller fluctuations in product output of the system.